Impact of obesity on the association between salt intake and blood pressure in adults with hypertension: findings from cross-sectional and longitudinal analyses

Background Obesity is a well-established risk factor for hypertension because excess adiposity contributes to increased blood volume, sympathetic activation, and altered renal sodium reabsorption, leading to elevated blood pressure (BP). Previously, obese individuals are theoretically believed to exhibit increased salt sensitivity. However, whether obesity affects the BP response to salt intake in daily living remains unclear. Purpose This study aimed to examine the effects of obesity on the association between salt intake and BP in adults with hypertension by cross-sectional and longitudinal analyses. Methods This observational study included adult males with hypertension who participated in a disease management program for cardiovascular risk factors using a mobile application. Daily salt intake was estimated based on urinary salt excretion using a validated self-monitoring device at home. Daily home BP was measured in the morning and evening, and morning systolic BP was analysed to avoid the influence of medications in this study. The mean values of salt intake and BP were calculated for seven consecutive days at baseline and after three months of the program. The cross-sectional analysis compared baseline BP across the tertiles of salt intake in those with and without obesity (body mass index [BMI] ≥ 25 kg/m2) separately. The longitudinal analysis compared the changes in BP across tertiles of salt intake reduction from baseline to after three months. Results This study included 1,095 participants with hypertension (median age; 57 years, median BMI; 26.4 kg/m2, obesity; 67.5%). The mean morning systolic BP at baseline was 131.0±11.5 mmHg. In the obesity group, more amount of salt intake was associated with higher BP (lowest tertile, estimated mean; 130.8 mmHg [95% confidence interval; 129.3–132.3], middle tertile; 132.1 [130.7–133.6], highest tertile; 133.4 [131.9–135.4], p=0.036) after adjusted for age, body weight, diabetes mellitus, and usage of BP medications (Figure 1). However, this relationship was not observed in the non-obese group (p=0.376). After three months, the participants’ BP decreased by a mean of 3.2±8.3 mmHg. Greater reduction of salt intake for three months in the longitudinal analysis was associated with greater BP reduction in the obesity group in the multivariate analysis (lowest tertile, estimated mean; -2.4 mmHg [95% confidence interval; -3.4–-1.4], middle tertile; -3.4 mmHg [-4.4–-2.4], highest tertile; -5.0 mmHg [-6.0–-3.9], p=0.001) (Figure 2). However, this relationship was not observed in the non-obese group (p=0.369). Conclusion The association between salt intake and BP was more pronounced in the obesity group cross-sectionally and longitudinally, suggesting the importance of salt reduction in BP control in individuals with hypertension and obesity.Figure 1.Blood pressure at baseline Figure 2.Blood pressure reduction